This is the first in a series of 4 posts whose goal is to briefly introduce rigid geometry with a focus on examples. The ultimate goal is to continue this post (and its sequels) with the reader hopefully having a broad grasp of the rigid geometry of the objects involved.

# Author: alexyoucis

# Objects with no rational models

The goal of this post, which is just a bit of fun between more serious posts/projects, is to discuss some examples of algebro-geometric objects over which have no models over smaller subfields and explain how moduli theory can help clarify their discovery in certain situations.

# A new paper draft

I have not been able to post of late as I’ve been quite busy working on several projects.

I wanted to make a post though discussing a new draft with my collaborator A. Bertoloni Meli that I’m quite excited about. In it we discuss a method for characterizing the local Langlands conjecture for certain groups as in Scholze’s paper [Sch]. Namely we show that for certain classes of groups an equation like that in the Scholze–Shin conjecture (see [Conjecture 7.1, SS]) is enough to characterize the local Langlands conjecture (for supercuspidal parameters) at least if one is willing to assume that other expected properties of the local Langlands conjecture hold.

The main original idea of this paper is the realization that while the Langlands–Kottwitz–Scholze method only deals with Hecke operators at integral level (e.g. see the introduction to [Sch]) that one can circumvent the difficult questions this raises (e.g. see [Question 7.5,SS]) if one is willing to not only consider the local Langlands conjecture for in isolation, but also the local Langlands conjecture for certain groups closely related to (so-called *elliptic hyperendoscopic groups*). Another nice byproduct of this approach is that while the Scholze–Shin conjecture is stated as a set of equations for all endoscopic triples for our paper shows that one needs only consider the trivial endoscopic situation (for elliptic hyperendoscopic groups of ).

This paper is closely related to the paper mentioned in this previous post where me and A. Bertoloni Meli discuss the proof of the Scholze–Shin conjecture for unramified unitary groups in the trivial endoscopic triple setting.

**References**

[Sch] Scholze, Peter. *The Local Langlands Correspondence for GL_n over -adic fields*, Invent. Math. 192 (2013), no. 3, 663–715.

[SS] Scholze, P. and Shin, S., 2013. On the cohomology of compact unitary group Shimura varieties at ramified split places. *Journal of the American Mathematical Society*, *26*(1), pp.261-294.

# Punctured disks and punctured curves and cohomology, oh my! (Part III: the affine curve case)

# Punctured disks and punctured curves and cohomology, oh my! (Part II: cohomology with supports)

# Punctured disks and punctured curves and cohomology, oh my! (Part I: the proper curve case)

This is the first in a series of posts whose goal is to compute the cohomology of the (several times) punctured closed -adic disk as well as the cohomology of smooth (and some non-smooth) algebraic curves.

# Weird example: finite maps don’t preserve projectivity

In this post we discuss a weird example of a finite map of varieties which doesn’t preserve projectivity.

# Classifying one dimensional groups (II)

In this post we classify one-dimensional connected group varieties of dimension .

# A smattering of representation theory

Attached below are notes that I wrote for a seminar at Berkeley.

The goal of the notes was to understand some of the representation theory surrounding Scholze’s paper on the cohomology of the Lubin–Tate tower. In particular I, Koji Shimizu, DongGyu Lim, and Sander Mack-Crane were/are interested in understanding whether there is a function field analogue of this paper.

In particular, it has an eye towards modular (i.e. mod ) representations of -adic groups. So, it discusses some of the classical theory of representations of -adic groups from a categorical perspective which serves one better in the modular representation setting. It also discusses the fascinating theorem of Kazhdan relating Hecke algebras for and where and are soemthing like the tilts of and .

Enjoy!

# I have a website now!

I mostly wanted to share for the implicit bragging that it only took me 9 hours (and the help of my girlfriend) to copy-and-paste enough basic HTML to make a simple website.